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Catalina/Capri 25/250 Sailor's Forums

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Props

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GaryB
Master Marine Consultant

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USA
4304 Posts

Initially Posted - 09/27/2008 : 16:07:22

I have basicly no reverse on my boat and I'm concerned I'm going to get caught in a situation where I need to stop relatively quickly to avoid something and I won't be able to. I can back out of the slip Ok but if I try to do it much more than idle the prop starts cavitating even though the cavitation plate is well below the surface. If moving forward even at idle and try to stop immediately get cavitation.

What pitch prop are you running and do you have any kind of reasonable reverse thrust?

Btw, I have a long shaft 8HP Suzuki 2 stroke.

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GaryB
Andiamo
'89 SR/WK #5862
Kemah,TX

Edited by - on

Chris Z
Captain

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452 Posts

Response Posted - 09/27/2008 : 17:09:30

Gary,
My prop is a very flat, high torque prop. I can get as much thrust in reverse as in forward.

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piseas
Former Treasurer

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USA
2017 Posts

Response Posted - 09/27/2008 : 20:40:53

Gary, your concerns are valid. Wondering if not job for local mechanic or perhaps time for new outboard.
Steve A

Edited by - on

GaryB
Master Marine Consultant

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USA
4304 Posts

Response Posted - 09/27/2008 : 20:59:39

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by piseas
 Gary, your concerns are valid. Wondering if not job for local mechanic or perhaps time for new outboard.
Steve A
<hr height="1" noshade id="quote"></blockquote id="quote">

I think I just need a prop with more pitch, the motor runs fine. I just don't know what pitch I need to get better reverse without losing too much forward thrust or RPM.

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 09/28/2008 : 07:19:04

Gary,

If your outboard has through-the-prop exhaust, your problem with operating in reverse may be caused by excessive through-the-prop exhaust gas that is causing cavitation. I don't know anything about Suzuki's, but is there some function on your outboard that redirects exhaust gases when in reverse?

If so, there might be a problem with it which is preventing the redirection of the exhaust gases causing the cavitation. This would mean you would have the same problem even if you changed props.

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by GaryB
 If moving forward even at idle and try to stop immediately get cavitation.<hr height="1" noshade id="quote"></blockquote id="quote">

Due to the relatively small outboards used, 6,000lb sailboats that are designed to move effortlessly through the water generally do not stop immediately but rather gradually come to a halt. Being a former powerboater, this was one of the first lessons I learned the hard way when I tried the old "power stop" once I was in the slip. I was taken completely by surprise how much momentum these boats carry!

Edited by - on

GaryB
Master Marine Consultant

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USA
4304 Posts

Response Posted - 09/28/2008 : 10:43:49

Don,

I do have thru the hub exhaust. I'll check with the dealer to see if there's some sort of device that might not be working properly.

I realize that a 6,000# boat will take a while to stop with an 8HP outboard but I can glide probably 200 yards in reverse and that's from 1 - 2 knots. The deceleration is almost imperceptible. I usually go to neutral while this far from my slip and then bump it in and out of gear to make the turn into the slip.

Now that 'I've been thinking about the thru the hub exhaust I think there may be an exhaust port in the area just below the cavitation plate (it's not the water intakes) that I see bubbling when I'm warming up the motor in the slip. Maybe that's where the cavitation is starting. I'll have to check it out this afternoon when I go down to the boat.

Edited by - GaryB on 09/28/2008 10:50:37

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 09/28/2008 : 11:18:23

>"I think I just need a prop with more pitch"

Th problem is usually too much pitch rather than too little. A 4 blade prop with low pitch and high blade area (often slightly larger diameter) is usually the ticket. Don't know about the availability of a 'high torque' prop for an older Suzuki... should be info online or at your dealer.

Most outboard props are designed to operate effeciently at speed... i.e. when pushing a boat on plane. Trying to shove a heavy displacement boat, if the prop is trying go 20 mph and the boat will only do 5... the prop tends to 'thrash' and ventilate. Kinda like being in the wrong gear when driving a car.

This is the problem that led to the development of 'high torque' outboards. Lower gear ratios and big, low-pitch props are optimized for pushing heavy non-displacement boats.

All that said, the ventilation problem Don mentions is common. Sucking exhaust gases through the prop instead of nice, clean water destroys reverse thrust. Many outboards have devices or strategies to overcome this... but some don't. Either your outboard has something that isn't working... or it doesn't have the 'anti-ventilation in reverse' features.

Most of the 2 strokes I've owned didn't have a reverse gear anti-ventilation devices... but that was a quite a few years ago.

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 09/28/2008 : 12:19:21

I think Clam is on the money... The redirection of exhaust in reverse came along in the newer Honda and Yamaha 4-strokes, along with their larger, lower-pitch "high thrust" props. I know my high-thrust Honda 8 can stop Passage almost like she's run into a pillow, whereas the older Honda 8 (standard size and pitch prop with no exhaust diversion) didn't come close. The diameter of the prop is limited by the space above the prop-shaft, which was increased in the newer models. I don't think you'll see the exhaust diverter in any 2-stroke.

The other factor is the depth of a long shaft (as opposed to X-long). Even if the anti-cavitation (or "anti-ventillation") plate is below the surface, if it isn't deep enough, the engine in reverse tends to create a vortex that sucks air down from the surface--you can hear the "giant sucking sound." If you don't have at least 4" of depth to the plate, you're especially subject to that when you gun it in reverse. (In forward, I suspect the bottom of the boat prevents it.)

The net (unfortunately) is that your 2-stroke Suzy might just be what it is. A different prop might help, but if it does, take it as a pleasant surprise. Just remember, "never approach a dock any faster than you're willing to hit it."

Edited by - Dave Bristle on 09/28/2008 12:22:02

GaryB
Master Marine Consultant

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USA
4304 Posts

Response Posted - 09/28/2008 : 14:10:26

I believe you guys are probably correct on the exhaust being sucked back into the prop. I've over the transom and the prop is totally wrapped in air bubbles at anything than just slightly above idle.

I believe I have a long shaft but the cavitation plate is at least 6"-10" below the surface.

Edited by - GaryB on 09/28/2008 14:11:01

OLarryR
Master Marine Consultant

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USA
3467 Posts

Response Posted - 09/28/2008 : 16:43:41

Gary,

I am not a prop tech specialist but something just does not seem right. I have a 9.9 hp Honda with a 4 blade prop but I remember way back in the 1980s when I had a 7.5 hp Evinrude 2 stroke with a 3 blade prop. Both fine in forward and in reverse. Your problem with reverse...is this something that just started happening or it's been that way all the time ? It would seem to me that if your outboard works fine in forward, then regardless of the blade pitch, it should be okay in reverse, maybe not same thrust but okay. But you mentioned going something like 200 yds at 1-2 knots while in reverse which almost seems like the prop is not turning or at least barely functioning ???

I would have to think that your outboard was designed satisfactoriy and so you are then dealing with either the outboard malfunctioning due to some internal deficiency or that cavitation is the culprit - Prop and exhaust ports are not sufficiently below the water surface. Not being sufficiently below may also be if for some reason when going in reverse, your outboard is shifting position a bit even though it should be locked. Have you observed any movement in the vertical alignment of the outboard from when you are going forward and then when you shift into reverse ? When going in reverse, the prop tends to thrust the outboard at the prop, more aft and if the outboard is shifting then the prop will come up a bit into more shallow water which could induce cavitation. Rcmd check to see what movement occurs in the outboard when shifting between forward and reverse. Also, if you are hearing the exhaust, then it would seem that the prop is too shallow.

Edited by - OLarryR on 09/28/2008 16:45:41

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 09/28/2008 : 16:58:46

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave Bristle
 I think Clam is on the money... The redirection of exhaust in reverse came along in the newer Honda and Yamaha 4-strokes,...<hr height="1" noshade id="quote"></blockquote id="quote">

My '95 Merc 8hp 2 stroke has through-the-prop exhaust in forward and when in reverse, exhaust gases are vented just under the cavitation plate forward of the prop on the starboard side of the lower unit.

Edited by - on

GaryB
Master Marine Consultant

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USA
4304 Posts

Response Posted - 09/28/2008 : 17:37:34

My cavitation plate is well below the water at all times and the motor is not shifting position when in reverse. It's been doing this since I bought the boat last October.

I think for some reason it's sucking exhaust gases back into the prop because of either a malfunction in the exhaust system itself or due to no exhaust redirection device on the motor. There are exhaust ports that are open where the water pump water comes out just below the cowling. Usually these should be enough when you're in reverse and water is stopping the thru hub exhaust. The only thing I can think is that these ports are partly blocked.

Edited by - on

Dave5041
Former Mainsheet Editor

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USA
3758 Posts

Response Posted - 09/29/2008 : 20:18:17

"The only thing I can think is that these ports are partly blocked."

Or the mechanism that causes the redirection (probably a mechanical link to the gearshift) is not functioning properly. I would probably take it to an authorized service center and explain the problem. My old Johnson redirected exhaust from the hub to the lower unit body in reverse and worked fine.

Edited by - on

DaveR
Master Marine Consultant

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USA
2015 Posts

Response Posted - 09/30/2008 : 08:11:27

I've got the same lack of reverse although I don't think as bad as Gary's. The prop is too small, the boat is too big, the pitch is too much and the wash can interact with the rudder in some circumstances in a unhelpful(?) way.
I just take my time and heed Dave B's observation ( "never approach a dock any faster than you're willing to hit it." )

Edited by - on

GaryB
Master Marine Consultant

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USA
4304 Posts

Response Posted - 09/30/2008 : 11:55:58

I went to the boat last night and took my camera but forgot to check the batteries before I left and of course they were dead.

On the bottom of the cavitation plate there is a 1" X 2" plate with holes in it. I believe it's a water intake since there are no intakes on the side of the lower unit like most outboards have.

Number 19 in this illustration -- http://store.brownspoint.com/dt8/fig020-8c8897.asp

There were bubbles coming from under the cavitation plate with the motor down all the way, in neutral with the motor idling. My guess is the exhaust is bubbling out around the joint where the prop meets the lower unit. It's not coming out of the back side of the prop hub. It's closer to the lower unit. There is also some exhaust coming out with the warm water exiting the engine just below the cowling.

With the boat tied to the dock I tried various RPM settings in both forward and reverse and could not get the prop to cavitate. I used normal power settings that I would use when backing up.

I'm not sure which prop I have since thewre are no markings on it but the only options are 3x9x7-1/4, 8-1/4, or 9-1/4. I assume the "9" is the diameter and last number indicates the pitch.

Edited by - GaryB on 09/30/2008 12:05:01

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 09/30/2008 : 12:51:58

Gary,

I experimented with props a few years ago on my 8hp 2 stroke Merc, replacing the original 9" x 9" prop with a 9 3/4" x 6.5" prop which had huge, round Mickey Mouse ear-like blades. I thought this low pitch prop would give me more bite around the docks, but since I use so little engine at manuevering speeds, this new prop gave me little, if any, advantage over the original higher pitch prop.

What the lower pitch prop did give me was horrid fuel economy and a lower top end speed. Needless to say, I soon switched back to the original prop.

Edited by - dlucier on 09/30/2008 13:57:52

dmpilc
Master Marine Consultant

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USA
4593 Posts

Response Posted - 09/30/2008 : 14:41:38

Item 19 in the drawing is the water intake. The cover plate with holes in it acts as a strainer.
My slip is about 8 spaces in from the end of the pier and I'm down to idle speed well before I make the turn to start going by the slips. When I get to the 1st slip, I pull the fuel hose. After that I shift to neutral. Only if I'm bucking a head wind do I need to give it an in gear boost.

Edited by - on

GaryB
Master Marine Consultant

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USA
4304 Posts

Response Posted - 09/30/2008 : 15:56:03

I think I'll order 50 HP Yamaha outboard. That should give me enough reverse thrust that I can stop on a dime if I need to avoid an idiot in front of me.

I guess I'll just quit worrying about it and deal with it like I have been for the past year. After all with my superior abilities I really don't need reverse anyway.

If I make a mistake (not likely!) I'll just go to full military power and add another slip to the dock!

Edited by - GaryB on 09/30/2008 15:56:46

DaveR
Master Marine Consultant

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USA
2015 Posts

Response Posted - 10/01/2008 : 10:49:23

Thanks for the prop info Don,
I was thinking about trying the "less pitch" approach, you saved me time and money. Why do we love this forum?

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